Recent studies of snakes have demonstrated their utility as models for investigating cardiovascular controls and designs. Recently, arterial baroreceptors were isolated to a discrete saccular outgrowth of the proximal pulmonary artery in a semiarboreal species. This structure is capable to control both systemic and pulmonary pressures because the systemic and pulmonary circulations are effectively linked in 'parallel' through intraventricular connections. Preliminary experiments suggest that the "baroreceptor sac" is the principal, if not sole, baroreceptive organ in this animal. The overall goal of the proposed project is to assess the morphology and function of this newly discovered organ, which, because of unusual properties and accessibility, can provide an important model to complement those studied in mammals. Baroreceptor sacs will be studied using light-, fluorescence- and electron-microscopic techniques to reveal their ultrastructural morphology and innervation. Morphology and compliance of the baroreceptive organ will be compared with that of nearby and associated vessels. The role of baroreceptors in cardiovascular control will be evaluated by quantifying cardiovascular responses to tilt, hemorrhage and pharmacologic disturbance of arterial pressure, using animals having intact or incapacitated baroreceptors (achieved by ligating or denervating the baroreceptor sac). Additional studies will examine the nature of cardiovascular responses elicited by baroreceptor nerve stimulation; in turn, afferent nerve activity will be recorded during the application of step pressure changes to the baroreceptors. The baroreceptive organ will be vascularly isolated in experiments intended to determine the open-loop gain of the baroreflex in conscious animals. Special emphasis will be given to evaluating the role of baroreceptors in controlling perfusion pressures of the pulmonary vasculature and in the long-term barostasis of both systemic and pulmonary circulations. Parallel studies will be conducted using an aquatic snake that differs in the importance of baroreflexes in regulating pulmonary perfusion during intermittent breathing. These investigations will further our understanding of baroreceptor physiology and evolution in tetrapod vertebrates. Because of the unique features of the ophidian baroreceptive organ, data from this project will have potentially important application to problems related to hypertension, pulmonary edema, long-term control and distribution of body fluids, and central integration of inputs from cardiovascular mechanoreceptors.